According to the Phase 2 of the Reformulated Gasoline (RFG) and Mobile Source Air Toxic (MSAT), programs, the actual benzene specification content in gasoline is less than 1 volume %; additionally, the Phase 3 of the RFG program and the California Air Resources Board (CARB) are established a gasoline benzene content up to 0.8% volume %. (http://www.mathproinc.com/pdf/2.1.4_Regulatory_Programs.pdf). Finally, the Environmental Protection Agency (EPA) is foreseeing regulations even more restricted before 2010, indicating the importance of gasoline benzene reduction.
In the gasoline mixture usually known as gasoline pool, approximately 81% of the benzene comes from reforming gasoline, 17% from FCC gasoline and 2% from other hydrocarbon streams, therefore, in order to comply with regulations and obtain a final product with less than 1 volume % of benzene, it is becoming a priority to reduce the benzene content from reformate.
Usually, if the processes used to reduce the benzene content are applied before the reforming process, they are called pretreatment processes, if they are used afterwards, they are denominated postreatment processes.
1) Pretreatment processes. This type of processes removes benzene precursors from the reforming feedstock. Therefore, the reformate obtained is produce with reduced benzene content. According to the state of the art, these types of processes are described in the patents U.S. Pat. No. 4,975,179 and U.S. Pat. No. 5,414,172.
2) Postreatment processes. The processes to reduce benzene by reformate postreatment are: hydrogenation, olefin alkylation and selective adsorption. Most of these postreatment processes involve two steps: first, a fractionation step is usually performed in order to obtain a benzene rich fraction (“hearthcut fraction”) followed by hydrogenation, alkylation or adsorption processes for final benzene reduction.
Among the processes involving hydrogenation postreatment are found in the patents U.S. Pat. No. 5,003,118, U.S. Pat. No. 5,254,763, U.S. Pat. No. 5,457,252, U.S. Pat. No. 5,817,227 and U.S. Pat. No. 6,048,450.
Most of the postreatment processes for reducing benzene contents in gasoline are based in alkylation with light olefins and an acid catalyst. Among the most recent patents considering this option are found in: U.S. Pat. No. 4,140,622, U.S. Pat. No. 4,209,383, U.S. Pat. No. 4,997,543, U.S. Pat. No. 5,053,573, U.S. Pat. No. 5,082,983, U.S. Pat. No. 5,120,890, U.S. Pat. No. 5,149,894, U.S. Pat. No. 5,185,486, U.S. Pat. No. 5,210,348, U.S. Pat. No. 5,273,644, U.S. Pat. No. 5,336,820, U.S. Pat. No. 5,347,061, U.S. Pat. No. 5,360,534, U.S. Pat. No. 5,380,425, U.S. Pat. No. 5,491,270 and U.S. Pat. No. 5,545,788. Most of this alkylation patents are associated to benzene fraction enrichment (“hearthcut fraction”).
Postreatment processes related to adsorption are further described.
The patents U.S. Pat. No. 5,186,819, U.S. Pat. No. 5,198,102 and U.S. Pat. No. 5,294,334 describe methods to selectively separate benzene from 27-191° C. gasoline fractions. Therefore, whereas the patent U.S. Pat. No. 5,186,819 describe a separation by selective adsorption, in patents U.S. Pat. No. 5,198,102 and U.S. Pat. No. 5,294,334 a benzene rich fraction (49 to 88° C. and 50 to 90° C., respectively) is obtained first by fractionated distillation, followed by a solid adsorbent selective adsorption of benzene. According to these patents a stream with minimum benzene content is obtained and the saturated adsorbent is regenerated by using a solvent capable of desorbing benzene. The use of zeolites X, Y and MOR interchanged with lithium, sodium, potassium rubidium or cesium, preferably NaX and NaY, but more preferably partial dehydrated zeolite NaY are reported in these patents. In the U.S. Pat. No. 5,186,819 and U.S. Pat. No. 5,198,102 patents, the desorbent used is toluene or xylene, preferably toluene. In the U.S. Pat. No. 5,294,334 patent, cyclohexane is used as desorbent. The desorbent is separated from benzene by distillation and is recycled to the adsorption zone. The processes of these referred patents can be carried out in a fixed-bed, simulated moving bed and magnetically stabilized bed.
A method for benzene selective separation and hydrogenation from naphtha boiling range hydrocarbon streams (27-191° C.) by passing this fraction trough a selective adsorbent bed and a hydrogenation catalyst, is described in U.S. Pat. No. 5,210,333 patent. Adsorbents used are zeolites X, Y and MOR interchanged with lithium, sodium, potassium rubidium or cesium, preferably NaX and NaY, but more preferably partial dehydrated zeolite NaY. Benzene is hydrogenated to cyclohexane by using a hydrogen activated catalyst, preferably with a platinum metal. This catalyst is supported over either the adsorbent or other material mixed with the adsorbent. The cyclohexane desorption is performed by a desorbent such as benzene, toluene, xylene, ethylbenzene or a mixture thereof. The patented process can be carried out in a fixed-bed, simulated moving bed and stabilized magnetically bed.
It should be noted that carbon molecular sieves or microporous carbon adsorbent (MCA) coming from pyrolysis of Saran were never used as adsorbent material in benzene separation.
Separation techniques based on adsorption processes, are known in the state-of-the-art, and among of them are temperature swing adsorption (TSA), pressure swing adsorption (PSA), elution or countercurrent chromatography, and the result of the combination thereof. These processes involve the contact of a liquid or gaseous mixture over a fixed bed of adsorbent in order to remove some of the components which can be adsorbed. Desorption can be carried out by various methods. Regeneration by pressure change is used in the PSD, an increment of temperature is employed to force desorption in a TDS process. The bed of adsorbent is regenerated by heating with pre-heated gas recirculation in an either opened or closed circuit, usually in the contrary direction which the adsorption was carried out. Other regeneration method by displacement involves the use of a desorbent which can be separated from the extract and the raffinate by distillation.
Benzene reduction by adsorption using the MCA prepared from the pyrolysis of the generically known copolymers of Saran is superior to prior technologies, which are known by the applicant. The adsorption process object of this invention reduce the benzene content from naphtha boiling range hydrocarbon streams (27 and 191° C.) by a two steps procedure: 1) separation by fractionation of C6's fraction, boiling point of 50 and 90° C. (benzene hearthcut fraction), and 2) adsorption of the benzene from the hearthcut fraction by passing this stream through a bed of MCA.
Therefore, is one object of the present invention to apply the highly selective microporous carbon adsorbent (MCA) for aromatic—non aromatic compounds separations, obtaining from the pyrolysis of the generically known Saran copolymers, in adsorption processes to reduce the benzene content in naphtha boiling range hydrocarbon streams.
Other object of the present invention is the application of the MCA in processes to produce gasoline with low benzene content from reforming and hydrocracked streams, cracked naphtha and hydrotreated cracked naphtha.
An additional object of the present invention is the application of the MCA to produce benzene free gasoline, which after being incorporated to the gasoline pool will produce gasoline with a benzene content of 1 volume % or less.
An additional object of the present invention is the application of the MCA to produce benzene with minimum purity of 99.8 weight % and maximum toluene content of 0.1 weight %, satisfying the ASTM D-2359-02 standard specifications for refined benzene-535.
The aforementioned and more objects in the present invention will be established in detail in the next chapters.